As shown in FIG. 1, the speed changing device of a conventional bicycle is provided with a guide seat 1 for guiding a drive chain 11, and with a pressing wheel 132 which is mounted on a threaded column 131 of a pressing frame 13 pivoted to a side frame 12. The pressing wheel 132 is intended to guide and press the drive chain 11. Such a prior art device for pressing a bicycle drive chain as described above is fixed and is therefore impractical.